1. Field of the Invention
The present invention relates to a communication terminal apparatus that is connected a single telephone subscriber line or a private branch exchange (PBX), and that is capable of automatically switching the telephone subscriber line or the PBX to either a telephone set or to a facsimile machine.
2. Description of the Related Art
Communication terminal apparatuses have been widespread which are connected either to an analog telephone line or to a PBX, and which are capable of sending and receiving facsimile messages. Such communication terminal apparatuses are designed to be connected to a telephone apparatus, and are capable of automatically identifying whether an incoming call is a voice call or a facsimile call so that a single input telephone line maybe shared between voice calls and facsimile calls.
A check may be made to determine whether the incoming call is a voice call or a facsimile call. Upon detection of a CNG signal (Calling Tone, the ITU-T Recommendation) having a frequency of 1100 Hz, which is on for one half of a second and off for three seconds, the apparatus enters a facsimile mode. If the CNG signal is not received, a ringer in the communication terminal apparatus or a ringer in the telephone rings to prompt the user to initiate a voice communication.
Japanese Patent Laid-Open No. 2002-281265 discloses the following technology for preventing an erroneous identification of the CNG signal or for alleviating a calling party's frustration.
(a) The apparatus of a called party sends a pseudo ring back tone which is a dummy ring back tone during the period in which an incoming CNG is watched for. Upon receiving a CNG signal components, the apparatus stops sending the pseudo ring back tone.
(b) If the apparatus does not receive a CNG signal within a predetermined time frame after having stopped sending the pseudo ring back tone in response to the detection of the CNG signal components, then the apparatus resumes sending the pseudo ring back tone.
(c) The period of time during which generation of the pseudo ring back tone is stopped may be selectable.
(d) The conditions under which the CNG signal components are detected may be selectable.
The aforementioned conventional communication terminal apparatus suffers from the following problems. If the called apparatus fails to receive the CNG signal after having stopped sending the pseudo ring back tone in response to the initial CNG signal components, the called apparatus resumes sending the pseudo ring back tone. Therefore, the pseudo ring back tone may not be sent at regular intervals, causing the caller to concern.
Additionally, the pseudo ring back tone may not be generated by the called apparatus in synchronism with the CNG signal. Therefore, the receiving apparatus may fail to detect the CNG signal for a while after it has begun to send out the pseudo ring back tone. Meanwhile, the calling party begins to be charged shortly after the called apparatus has established the dc loop of the called party's telephone line, so that the calling party is charged for a period during which the CNG signal is being watched for. As a result, the longer the CNG signal verification time, the more uneconomical to the caller's apparatus.